The amount of soot formed when burning a benzene/hexane mixture in a turbulent combustor was examined. Soot concentration profiles in the same combustor for kerosene fuel are given. The chemistry of the formation of soot precursors, the nucleation, growth and subsequent burnout of soot particles, and the effect of mixing on the previous steps were considered

Bittner, J. D.

Howard, J. B.

Neoh, K.

Prado, B.

English

NASA Technical Reports Server

SOOT FORMATION AND BURNOUT 
G, Prado, J.D. B i t t n e r ,  K. Neoh, 
Massachusetts I n s t i t u t e  of 
I N  FLAMES 
and J.B. Howard 
Technology 
Foreseeable t rends  i n  f o s s i l  f u e l  a v a i l a b i l i t y  i n d i c a t e  t h a t  t he  aromatic 
content of f u t u r e  f u e l s  w i l l  be s i g n i f i c a n t l y  higher than present levels. 
There are s u b s t a n t i a l  economic and public hea l th  incent ives  t o  pursue the  c lean  
and e f f i c i e n t  use of such h ighly  aromatic f u e l s ,  e s p e c i a l l y  as t h e i r  s t rong  
soot ing  tendency is w e l l  known. For example, t h e  Experimental Referee Broad 
Spec i f i ca t ion  (ERBS) f u e l ,  proposed by NASA as r ep resen ta t ive  of a near-term 
a l t e r n a t e  f u e l  f o r  a i r c r a f t  conta ins  29.6% of aromatics,  as compared t o  17.3% 
i n  Jet A a v i a t i o n  tu rb ine  kerosene. 
The amount of soot  formed when burning a benzene/hexane mixture i n  a 
turbulen t  combustor simulating gas turb ine  engines i l l u s t r a t e s  t he  expected 
e f f e c t  of increas ing  t h e  aromatic f r a c t i o n  (Fig. 1). Soot concent ra t ion  
p r o f i l e s  i n  the  s a m e  combustor f o r  kerosene f u e l  are shown on Fig. 2. Modeling 
these  p r o f i l e s  r equ i r e s  t he  understanding of complex processes such as t h e  
chemistry of formation of soot precursors,  t h e  nucleation, growth and subse- 
quent burnout of soo t  particles, and the  e f f e c t  of mixing i n t e n s i t y  on the  
previous s t eps .  The p resen ta t ion  w i l l  focus on recent  r e s u l t s  from experi- 
ments designed t o  shed l i g h t  on these d i f f e r e n t  processes 
While t h e  s t rong  soot ing  tendencies of aromatic hydrocarbons are w e l l  
known, the  underlying mechanism of t h i s  behavior is  not es tab l i shed .  some 
experimental evidence suggests t h a t  t h e  i n t a c t  aromatic r i n g  and not  fragmen- 
t a t i o n  products is respons ib le  f o r  t h e  propensity of aromatic hydrocarbons t o  
form soot. Accordingly, des t ruc t ion  of t h e  r i n g  during combustion might o f f e r  
a route  f o r  t he  reduction of soo t  formation from aromatic fue l s .  To inves t i -  
g a t e  these  mechanisms, t h e  s t r u c t u r e  of a near sooting, low pressure  (20 t o r r s )  
benzene-oxygen-argon flame has  been s tudied  using a m a s s  spectrometer coupled 
with a molecular beam sampling systeml. Some r e s u l t s  i n  t e r m s  of mole f r a c t i o n  
of molecular and r a d i c a l  species are shown i n  Figs. 3 t o  6 ,  and w i l l  be  
discussed i n  more d e t a i l s .  They suggest t h a t  soo t  precursors are high molecular 
mass polyaromatic molecules. The process may s tar t  by add i t ion  of vinyl- 
ace ty lene  t o  phenyl r a d i c a l  t o  form naphtalene (Table I). 
r a d i c a l  is formed by methyl r a d i c a l  s u b s t i t u t i o n a t  t h e  a-posit ionin naphthalene 
(Table 11) and subsequent a b s t r a c t i o n  of a methyl hydrogen. 
C2H2 add i t ion  t o  t h i s  benzyl-type r a d i c a l  can be r ap id ly  s t a b i l i z e d  by 
cyc l i za t ion  t o  form a six-membered r i n g  through an i n t e r n a l  aromatic 
s u b s t i t u t i o n  reac t ion .  
tendencies of methyl-substituted aromatics under a v a r i e t y  of conditions.  
A benzyl-type 
The product of 
Such a mechanism w i l l  expla in  the  s t r o n g  sooting 
The na tu re  of soot  n u c l e i  i s  not  w e l l  e s tab l i shed .  A p o s s i b i l i t y  is  t h a t  
t he  l a r g e  polyaromatic molecules undergo coagulation, and once formed, t he  
p a r t i c l e s  provide a su r face  on which gas phase material can deposit .  
r e s u l t s 2  derived from l a s e r - l i g h t  s c a t t e r i n g  and absorp t ion  study of soot  
Recent 
131 
https://ntrs.nasa.gov/search.jsp?R=19800020819 2020-03-21T03:28:37+00:00Z
formation i n  premixed and d i f fus ion  laminar flames r evea l  two regimes of soot  
p a r t i c l e  growth. 
particles maintain an approximately sphe r i ca l  shape. 
f r ac t ion ,  chain forming c o l l i s i o n s  occur, simultaneously with some su r face  
growth. I n  both regimes, t he  p a r t i c l e  number dens i ty  i s  described by the  f r e e  
molecular c o l l i s i o n  theory, t he  f i n a l  number of agglomerates being always c lose  
' to 1010/cm3 (Fig. 7 ) .  
For soo t  volume f r a c t i o n  smaller than about t he  growing 
A t  l a r g e r  soot  volume 
I n  most p r a c t i c a l  combustors, soo t  burnout occurs a t  t he  e n d o f t h e  process, 
t h e  amount of soot re leased  i n  the  environment (Fig. 2 ) .  An ongoing reducin3 program a t  M.I.T. i s  addressing t h e  following questions 1) relative importance 
of t h e  var ious  oxidants 02,  OH, 0, C02, and H20 i n  soo t  oxidation, 2) e f f e c t  of 
oxidation on the i n t e r n a l  s t r u c t u r e  of t h e  soot p a r t i c l e s ,  3 )  burnout charac- 
terist ics of soo t  from premixed flames and from f u e l  pyro lys i s ,  4) e f f e c t  of 
temperature-time h i s t o r y  of soo t  formation on soot  burnout. Results a t  t h i s  
s t age  i n d i c a t e  t h a t  t he  s p e c i f i c  burnout rates based on the  ex te rna l  sur face  
area of t he  p a r t i c l e s  are l a r g e r  than predic ted  by the  classical Nagle and 
Strickland-Constable formula. The N2 BET sur face  area of soot  increases  
considerably more during burnout than can 'be  accounted f o r  by the  decrease of 
par t ic le  diameter, i nd ica t ing  opening of porosity.  Under some conditions,  a 
breakup of soo t  agglomerates appears t o  occur during t h e  post-combustion. 
F ina l ly ,  i n  tu rbulen t  combustors4 , the  mixing c h a r a c t e r i s i t i c s  of f u e l  and 
a i r  can a f f e c t  considerably t h e  amount of soot produced and re leased  i n  the  
atmosphere (Fig. 2 and 8 ) .  For some conditions (Fig. 8) t he  soot  emission 
leve ls  a r e  i n  reverse order of maximum soo t  concentration, s t r e s s i n g  the  impor- 
tance of burnout. 
REFERENCES 
1. B i t t n e r ,  J .D. :  Ph.D. Thesis, M.I.T. ( i n  preparation) 
2. Prado, G. e t  a l :  Submitted f o r  Presenta t ion  a t  t h e  18th  Symposium ( In t e r -  
na t iona l )  on Combustion, Waterloo (Canada) 
3 .  Neoh, K.:  Ph.D. Thesis,  M.I.T. ( i n  preparation) 
4. Prado, G. e t  a l :  16 th  Symposium ( In t e rna t iona l )  on Combustion, p.649(1977) 
132 
f 
TABLE 11: METHYL SUBSTITUTION 
133 
Weight o/o of Benzene 
FIG. 1 INFLUENCE OF FUEL AROMATIC CONTENT ON SOOT CONCENTRATION .~ -TURBULENT COMBUSTOR. FUEL EQUIVALENCE RATIO = 1.0. ATOMI- 
ZING AIR PRESSURE = 184kPa. COLD GAS VELOCITY = 2.67 m/sec. (e) MIXTURE HEPTANE-BENZENE . (A) KEROSENE 
I o4 
IO' 
1 0; 
10 
0 10 20 30 40 50 60 70 
DISTANCE FROM NOZZLE (cm) 
FIG. 2 INFLUENCE OF ATOMIZING-AIR PRESSURE ON AXIAL SOOT CONCEN- 
TZATION KEROSENE/AIR. FUEL EQUIVALENCE RATIO = 1.0. COLD 
GAS VELOCITY = 0.96m/sec. ATOMIZING-AIR PRESSURE = 
(0) 184kPa. (01  205kPa, (A) 239kPa. 
-
134 
F I G .  - 
0.4 
0 
& 0.3 
z 
4 
(L 
IL 
,0.2 
75 
2 
0. I 
I 
m 
0 
0 
a 
x 
z 
I- u 
(L 
IL 
w 
_I 
0 
2 
0 
8 
6 
4 
2 
0 2 3 4  5 6 7 8 9 IO II  121314 
TIME. ms 
800 
600 
400 5 w 
2002 
(L 
000 
800 5 
I- 
600 ’ 
400 
- 3 MOLE FRACTION AND TEMPERATURE VS DISTANCE FROM BURNER I N  
A NEAR SOOTING( 4 = 1.8)8BMZENE(13.5m%)OXYGEN(56.5m%)ARGON 
(30 .0m%)  FLAME. COLD GAS VELOCITY O.Sm/sec. PIieSSURE = 
2 . 6 7 k P a ( Z O T O R R ) .  (A) MAJOR STABLE S P E C I E S .  (B)  H,OH,HOZ AND 
TEMPERATURE 
I 
I I  I I I 
0 2 3  4 5 6 
TIME, ms 
E FLUX AND MOLE FRACTION O F  BENZENE AND MOW FRACTIONS 
OF S P E C I E S  O F  POSSIBLE IMPORTANCE I N  BCNZENE DECOMPOSITION 
VS DISTANCE FROM BURNER I N  A NEAR SOOTING (-$ = 1.8)BENZENE 
( 1 3 . 5 m % )  OXYGEN (56 .5m%)  ARGON ( 3 0 . h % )  FLAME. COLD GAS VELO- 
C I T Y  = O.Sm/sec. PRESSURE= 2 . 6 7 k P a  (ZOTORR) 
135 
IO 20 30 40 
DISTANCE FROM BURNER, m m  
RELATIVE INTENSITIES OF SIGNALS FROM HIGH MASS MODE OPERA- 
TION VS DISTANCE FROM BURNER. BENZENE-OXYGEN-ARGON FLAME. 
PRESSURE = 2.67kPa (20 TORR), COLD GAS VELOCITY O.Sm/sec. 
O.! 
I O  
z 
I- 
(r 
LL 
0 
: IO 
w 
_I 
0 z 
IO 
10 
BLUE ORANGE 
DISTANCE FROM BURNER, mm 
FIG. 6 MOLE FRACTION OF C H2,02,and C6H VS DISTANCE FROM BURYER - FOR NEAR SOOTING (a = 1.8, OPEN 2YMBOLS) AND SOOTING 
( '$ = 2.0, SHADED SYMBOLS) FLAMES. BENZENE-OXYGEN-ARGON 
FLAME. PRESSURE = 2.67kPa (20 TORR). COLD GAS VELOCITY = 
O.Sm/sec. 
136 
N,cm3 
H.A.B. (mm) 
!ZZZRjEZ:% 1.) AND COMPUTED (e) NUMBER DENSITY O F  PARTI- 
RATIO = 2 .5 .  ATMOSPHERIC PRESSURE. COLD GAS VELOCITY = 
) f PROPANE/OXYGEN FLAME. FUEL EQUIVALENCE 
S.SOnn/sec. 
I I I I 
40 5 0  0 10 2 0  30 
D I S T A N C E  FROM N O Z Z L E  ( C M )  
AXIAL SOOT CONCENTRATIONS AT TEREE ATOXIZING-AIR PRESSURE 
KXROSENE/AIR- FUEL ERUIVAGENCE RATIO = 1.0.  COLD GAS 
VELOCITY - 2.67daeo. ATOMIZIEIG-AIR PFSSSURE - ( 0 ) 1 8 4 k P a ,  
(DIZOSkPa, IN239kPa.  
137 


Soot formation and burnout in flames

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